1. Field of the Invention
The present invention relates to an image processing device and an image processing method for discriminating a line-like structure or a plate-like structure in an image, as well as a program for causing a computer to carry out the image processing method.
2. Description of the Related Art
Along with advancement of medical devices (such as multiple-detector CT) in recent years, high-quality three-dimensional images are used in imaging diagnosis. Such a three-dimensional image is formed from a number of two-dimensional images and thus has a large amount of information. Therefore, it may take time for a doctor to find a desired part to be observed and diagnose the part. To address this problem, it has been practiced to extract an organ of interest and display it in MIP, VR, CPR, or the like, to enhance visibility of the entire organ and a lesion, thereby improving efficiency of the diagnosis.
On the other hand, as a technique to extract a blood vessel and a bone in a medical image, Hessian analysis using a Hessian matrix has been proposed (see A. F. Frangi et. al., “Multiscale vessel enhancement filtering”, MICCAI, Vol. 1496, pp. 130-137, 1998, which will hereinafter be referred to as Non-Patent Document 1). The Hessian analysis is used to analyze eigenvalues of a Hessian matrix, which contains, as its elements, second order partial derivatives calculated using a second order derivative kernel of a predetermined filter, such as a Gaussian kernel, to discriminate whether a local structure in the image is a point, a line or a surface. Use of the Hessian analysis allows discrimination of a blood vessel as a “line-like structure” and a bone as a “plate-like structure”.
However, since blood vessels and bones have various thicknesses and sizes, discrimination of the blood vessels and bones may not be achieved only by using the Hessian analysis. For example, in the case where a cortical bone, such as the spine, is extracted as the plate-like structure, if a filter kernel having a size enough to detect the surface portion of the cortical bone is used, not only the plate-like structures of a bone 100 but also the surface of a thick blood vessel 102 may be discriminated as the plate-like structure, as shown by the hatched areas in FIG. 8. This problem occurs because the second order partial derivatives are high in a direction perpendicular to the surface of the thick blood vessel and are low in two directions parallel to the surface of the blood vessel, and thus the thick blood vessel shows the same characteristics as that of the plate-like structure. In contrast, in the case where a blood vessel is extracted as the line-like structure, if a filter kernel having a size enough to detect the line-like portion of the blood vessel is used, not only the portion of the blood vessel 102 forming the line-like structure but also the corner portions of the bone 100 may be discriminated as the line-like structure, as shown at the hatched areas in FIG. 9.